The present invention relates to a continuous drum dryer mixer system, and especially to a system for heating and drying virgin aggregates, as well as recycled asphaltic pavement materials and mixing such with liquid asphalt cement in the preparation of asphalt paving materials.
In the past, asphalt plants have generally been of two types, the Batch type, and more recently the Drum Mix type. Batch type plants typically have hopper feeds for feeding different sizes of unweighed cold aggregate materials by means of a cold feed conveyor into the dryer. The dryer typically involves a large elongated drum having a burner directed into one end, with the aggregate being fed therethrough from the other end, and providing rotation of the drum with flights for lifting the aggregate material and dropping it through the hot gases and causing it to be dried. As the material passes through the drum, the hot air and gases pass out of the other end of the dryer into a ducting system which is directed towards a wet wash, mechanical collector, or fabric filter dust collector for cleaning the air and gases prior to their discharge from a stack. Meanwhile, the dried aggregate materials are fed up a bucket type hot elevator to a batching tower, where the material is passed through screens for separating it into sizes and storing it into separate compartments whence it is withdrawn pursuant to various formulas into the weigh box. After weighing, the aggregates are dropped into a mixer at the bottom of the tower where the exact amount of hot liquid asphalt is added, along with mineral fillers, and mixed to specified time durations, prior to loading into a truck. This type of plant has been the most commonly used around the world, but has the disadvantages of not being able to process recycled asphaltic pavement materials.
The drum mix plant has a feed system which preproportions the ratios of the various aggregates on to the cold feed conveyors, feeds it into a rotating elongated drum dryer where a burner is directed into the same end as the cold aggregates. As the aggregates tumble and progress through the rotating drum dryer they absorb heat and shed moisture. Half-way into the drum liquid asphalt is injected which mixes with the aggregates through the tumbling action and exit the dryer discharge as a finished mixture ready to be loaded into trucks from hauling to the paving site. This type of drum mix plant is most commonly used to process some ratio of recycled asphaltic pavement (RAP) which is introduced into the elongated drum dryer at about mid-point. The disadvantage of this drum mix plant is that since both the liquid asphalt, as well as the recycled asphaltic pavement are introduced into the same single elongated drum dryer into which the intense fire is released from the burner, and subjected to the high velocity of the plant exhaust system. The high temperature from the flame and combustion gases cause burning of the liquid asphalt, degradation of quality of the finished product, release of volatiles from the liquid asphalt, as well as the asphaltic pavement materials, generation of blue smoke and other severe pollutants, release of odors, and also can cause fires and explosions from the gasified volatiles which tend to recondense into liquid from in the plastic filters of the exhaust system.
Prior U.S. Pat. No. 4,262,429 deals with a method of and apparatus for drying materials which uses coaxial cylinders with one being fed moist fine aggregates and the other being fed moist course aggregates for mixing and drying the aggregates. It does not deal with the manufacture of paving asphalt nor the use of recycled asphalt and does not provide the pollution reduction (for asphalt plants) accomplished with an exhaust feedback loop and dust collection chamber.
The present invention is directed to an asphalt plant which can process a mixture of recycled asphalt and virgin materials or can process only virgin materials, without oxidizing, distilling or burning the liquid asphalt, or that of the recycled asphaltic paving material; and without releasing volatile matter, particulate pollutants, or odors. The plant utilizes the waste heat in the system for controlling the temperature of the product, and contains and incinerates any vapors which may be released by the finished product.
It is accordingly, an aim of the present invention to provide an aggregate and virgin asphalt plant which can utilize recycled asphalt materials and produce a superior product utilizing substantial amounts of normally wasted energy, and dramatically reduce release of environmental pollutants, as well as reduce fire and explosion hazards.